Don't allow use of the default debian user account

src/freedombone-config

       echo 'No user account was selected'
       exit 8347
   fi
+  if [[ $MY_USERNAME == 'debian' ]]; then
+      echo "Don't use the default debian user account"
+      exit 9341
+  fi
   if [ ! -d /home/$MY_USERNAME ]; then
       echo "The directory /home/$MY_USERNAME does not exist"
       exit 6437